The bouncy house seems like an innocuous childhood delight. What’s not to love about a giant, inflatable room where kids can jump and scream in one safely contained location? The problem, recounted in a dubious study by a team of geographers and doctors, is that bouncy houses can cause heat stroke, especially this summer during our hottest year on record.

Researchers from the University of Georgia and the Dell Children’s Medical Center in Austin, Texas knew that bouncy castles caused an enormous number of injuries. But over the past 20 years, the numbers have skyrocketed. As the researchers write in a paper out today in Bulletin of the American Meteorological Society, “One of the most staggering findings is that during the period of 1995 to 2010, a 15-fold increase was observed in the rate and number of bounce house injuries (roughly 5.28 injuries per 100 000 children in the United States annually).” Staggering, indeed. Mostly these were fractures, strains, and “other injuries to the upper and lower extremities.”

The researchers decided to find out whether heat stroke was another, hidden danger to be found in these funhouses of pain. Though they could find only one reported instance of heat stroke from a bouncy castle, they forged ahead with their quest. Last summer, they spent a single afternoon measuring the “air temperature, humidity, wind speed, and computed heat index values” inside a typical bouncy castle, which they inflated in a grassy plaza on the University of Georgia campus. What they discovered is that these puffy joy rooms are actually heat-trapping danger chambers: